30th April 2024

A 58-year-old man presents with a 3-month history of progressive fatigue and weakness.

A 58-year-old man presents with a 3-month history of progressive fatigue and weakness. He is a strict vegan and does not take any supplements. He has no significant past medical history. His diet mainly consists of grains and vegetables, and he denies alcohol use.

Investigations:

Haemoglobin: 98 g/L (130–180)

MCV: 110 fL (80–96)

White cell count: 4.2 × 10⁹/L (4.0–11.0)

Platelets: 150 × 10⁹/L (150–400)

Serum vitamin B12: 120 picomols/L (148–600)

Serum folate: 6 ng/mL (3–13)

Which further investigation is most appropriate to confirm the diagnosis?